RFC 1476 RAP June 19931.1 Link-State and Distance-Vector
Of the two major classes of routing algorithm, link-state and
distance vector, only distance vector seems to scale from the local
network (where RIP is existence-proof of its validity) to large scale
inter-domain policy routing, where the number of links and policies
exceeds the ability of each router to map the entire network.
In between, we have OSPF, an open link state (specifically, using
shortest-path-first analysis of the graph, hence the acronym)
protocol, with extensive development in intra-area routing.
Since distance vector has proven useful at both ends of the range, it
seems reasonable to apply it to the entire range of scales, creating
a protocol that works automatically on small groups of LANs, but can
apply fairly arbitrary policy in the largest networks.
This helps model the real world, where networks are not clearly
divided into hierarchical domains with identifiable "border" routers,
but have many links across organizational structure and over back
fences.
1.2 Terminology
The RAP protocol propagates routes in the opposite direction to the
travel of datagrams using the routes. To avoid confusion explaining
the routing protocol, several terms are distinguished:
source where datagrams come from, the source of the
datagrams
destination where datagrams go to, the destination of the
datagrams
origin where routing information originates, the router
initially inserting route information into the
RAP domain
target where routing information goes, the target uses the
information to send datagrams
1.3 Philosophy
Protocols should become simpler as they evolve.
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RFC 1476 RAP June 19932. RAP Protocol
The RAP protocol operates on TCP port 38, with peers opening a
symmetric TCP connection between the RAP ports on each system. Thus
only one RAP connection exists between any pair of peers.
RAP is also used on UDP port 38, as a peer discovery method. Hosts
(i.e., non-routing systems) may listen to RAP datagrams on this port
to discover local gateways. This is in addition to, or in lieu of,
an Internet Standard gateway discovery protocol, which does not exist
at this writing.
The peers then use RAP commands to send each other all routes
available though the sending peer. This occurs as a full-duplex
(i.e., simultaneous) exchange of information, with no acknowledgement
of individual commands.
Once the initial exchange has been completed, the peers send only
updates to routes, new routes, and purge commands to delete routes
previously offered.
When the connection is broken, each system purges all routes that had
been offered by the peer.
2.1 Command Header Format
Each RAP command starts with a header. The header contains a length
field to identify the start of the next packet in the TCP stream, a
version number, and the code for the command. On UDP, the length
field does not appear: each UDP datagram must contain exactly one
RAP command and not contain data or padding after the end of the
command.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RAP version | command code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.1.1 Length field
The length is a 32 bit unsigned number specifying the offset in bytes
from the first byte of the length field of this command packet to the
start of the length field of the next. The minimum value is 8.
There is no specific limit to the length of a command packet;
implementations MUST be able to at least count and skip over a packet
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RFC 1476 RAP June 1993
that is too large and then MAY send an error indication.
Each version of the protocol will profile what size should be
considered the limit for senders, and what (larger) size should be
considered by receivers to mean that the connection is insane:
either unsynchronized or worse.
For version 1 of the protocol, senders MUST NOT send command packets
greater than 16384 bytes. Receivers SHOULD consider packets that
appear to be greater than 162144 bytes in length to be an indication
of an unrecoverable error.
Note that these limits probably will not be approached in normal
operation of version 1 of the protocol; receivers may reasonably
decline to use routes described by 16K bytes of metrics and policy.
But even the most memory-restricted implementation MUST be able to
skip such a command packet.
2.1.2 RAP version
The version field is a 16 bit unsigned number. It identifies the
version of RAP used for that command. Note that commands with
different versions may be mixed on the same connection, although the
usual procedure will be to do the serious protocol (exchanging route
updates) only at the highest version common to both ends of the
connection.
Each side starts the connection by sending a poll command, using the
highest version supported and continues by using the highest version
received in any command from the remote. The response to the poll
will either be a no-operation packet at that version or an error
packet at the highest version supported by the remote.
This document describes version 1 of the RAP protocol.
2.2 RAP Commands
There five simple RAP commands, described in the following sections.
2.2.1 No operation
The no operation command serves to reset the poll timer (see next
section) of the receiver, or (as a side effect) to tell the receiver
that a particular version is supported. It never contains option
specific data and its length is always 8.
The no operation command is also used in a UDP broadcast to inform
other systems that the sender is running RAP actively on the network
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RFC 1476 RAP June 1993
and is both a possible gateway and a candidate peer. If this command
is being sent in response to a broadcast poll, it should be sent only
to the poller.
A RAP process may send such broadcasts in a startup sequence, or it
may persist indefinitely to inform other systems coming on line. If
it persists, it must not send them more than once every 10 minutes
(after the initial startup sequence). If the RAP process sends polls
as part of its startup, it must not persist in sending them after the
startup sequence.
The command code for no-operation is always 0, regardless of RAP
version.
2.2.2 Poll
A poll command packet requests that the other side transmit either a
no-operation packet, or some other packet if sent without delay.
(i.e., receivers MUST NOT delay a response to a poll by waiting for
some other packet expected to be queued soon.)
The poll command code is always 1, regardless of version, and the
length is always 8.
Each RAP implementation runs a timer for each connection, to ensure
that if the other system becomes unreachable, the connection will be
closed or reset. The timers run at each end of the connection are
independent; each system is responsible for sending polls in time to
reset its own timer.
The timer MUST be reset (restarted) on the receipt of any RAP packet,
regardless of whether the version or command code is known.
In normal operation, if route updates are being sent in both
directions, polls may not be necessary for long periods of time as
the timers are continually reset. When the connection is quiescent,
both timers will typically get reset as a result of the side with the
shorter timer doing a poll, and then getting a no-operation in
response. RAP implementations MUST NOT be dependent in any way on
the size or existence of the remote timer.
An implementation that has access to information from the TCP layer,
such as the results of TCP layer keepalives, MAY use this instead of
or in addition to a timer. However, the use of TCP keepalives is
discouraged, and this procedure does not ensure that the remote RAP
process is alive, only that its TCP is accepting data. Thus a
failure mode exists that would not exist for active RAP layer polls.
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RFC 1476 RAP June 1993
The timer MUST be implemented, SHOULD be configurable in at least the
range 1 to 10 minutes on a per-peer basis, and MAY be infinite
(disabled) by explicit configuration.
On UDP, a system (router or non-routing host) may send RAP polls to
attempt to locate candidate peers or possible gateways. Such a
system must not persist in sending polls after its startup sequence,
except that a system which actually has offered traffic for non-local
destinations, and has no available gateways, may continue to send
periodic polls to attempt to acquire a gateway.
2.2.3 Error
The error packet is used to report an error, whether fatal, serious
or informational. It includes a null terminated text description in
ISO-10646-UTF-1 of the condition, which may be useful to a human
administrator, and SHOULD be written to a log file. (The machine is
not expected to understand the text.)
Errors are actual failures (in the interpretation of the receiver) to
use the correct syntax and semantics of the RAP protocol itself, or
"failure" of the receiver to implement a version of the protocol.
Other conditions that may require action on the part of the peer
(such as purging a route) are given their own command codes.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RAP version (1) | command code (2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| error code (0) [reserved] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| description |
+ +
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The RAP system receiving an Error packet MUST NOT regard it as fatal,
and close the connection or discard routes. If the sending system
desires the condition to be fatal (unrecoverable), its proper action
is to close the connection. This requirement is to prevent the kind
of failure mode demonstrated by hosts that killed off TCP connections
on the receipt of ICMP Host-Unreachable notifications, even when the
condition is transient. We do not want to discourage the reporting
of errors, in the way that some implementations avoided sending ICMP
datagrams to deal with overly sensitive hosts.
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RFC 1476 RAP June 1993
An error packet MUST NOT be sent in response to something that is (or
might be) an error packet itself. Subsequent versions of RAP should
keep the command code point (2) of the error packet.
2.2.4 Add Route
The add route command offers a route to the receiving peer. As noted
later, it MUST be a route actually loaded into the forwarding
database of the offering peer at the time the add route is sent.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RAP version (1) | command code (3) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| distance | (MBZ) | mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| destination network |
+ +
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| route identifier |
+ +
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| metrics and options .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The add route command describes a single offered route, with the
metrics and other options (such as policies) associated with the
route.
Distance is a simple count of the hops to the RAP process (or other
routing process) that originated the route, incremented every time
the route is forwarded. Its initial value may be greater than 1,
particularily for a route that is administratively configured to
aggregate routes for a large network or AD. It may also enter the
RAP routing domain for the first time with a non-zero distance
because the route originated in RIP, OSPF, or BGP; if so, the
distance carried in that protocol is copied into the RAP route.
The mask is a count of the number of bits of prefix ones in the
binary representation of the network mask. Non-contiguous masks are
not supported directly. (The destination restriction option may be
used to give another, non-contiguous, mask; the header mask would
then describes the number of contiguous ones.)
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RFC 1476 RAP June 1993
The route identifier is a 64 bit value that the IP forwarding module
on the sending host can use to rapidly identify the route and the
next hop for each incoming datagram. The host receiving the route
places this identifier into the forward route ID field of the
datagrams being sent to this host.
The route ID is also used to uniquely identify the route in the purge
route operation.
2.2.5 Purge Route
The purge route command requires that the receiving peer delete a
route from its database if in use, and requires that it revoke that
route from any of its peers to whom it has offered the route. This
command should preferably be sent before the route is deleted from
the sending peer's forwarding database, but this is not (cannot be)
required; it should be sent without delay when the route is removed.
The command code is 4. The format is the same as add route without
any added metrics or options.
If the route identifier in a purge route command is zero, the command
requires the deletion of all routes to the destination previously
offered by this peer.
3. Attributes of Routes
There are a rather large number of possible attributes.
Possibilities include both metrics, and other options describing for
example policy restrictions and alterations of proximity. Any
particular route will usefully carry only a few attributes or none at
all, particularily on an infrastructure backbone. A reasonable
policy for the routers that make up a backbone might be to strip all
attributes before propagating routes (discarding routes that carry
attributes with class indications prohibiting this), and then adding
(for example) an AUP attribute to all routes propagated off of the
backbone. A less drastic method would be to simply prefer routes
with no restrictions, but still propagate a route with restrictions
if no other is available.
Most options can occur more than once in a route if there is any
sensible reason to do so.
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RFC 1476 RAP June 19933.1 Metric and Option Format
Each metric or option for a route begins with a 32 bit header:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length | C | format | type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| option data ... | padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RAP Option/Metric Header Format
A description of each field:
length length of the option or metric
C option class, see below
format data format
type option type identifier
data variable length
3.1.1 Option Class
This field tells implementations what to do with routes containing
options or metrics they do not understand. No implementation is
required to implement (i.e., understand) any given option or metric
by the RAP specification itself, except for the distance metric in
the RAP header.
Classes:
0 use, propagate, and include this option unmodified
1 use, propagate, but do not include this option
2 use this route, but do not propagate it
3 discard this route
Note that class 0 is an imperative: if the route is propagated, the
option must be included.
Class and type are entirely orthogonal, different implementations
might use different classes for the same option or metric.
3.1.2 Type
The type code identifies the specific option or metric. The codes
are part of the option descriptions following.
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RFC 1476 RAP June 1993
Type 0 indicates a null (no-operation) option. It should be class
zero, but an implementation that "understands" the null option may
decline to propagate it.
Note that since an implementation may delete an option of class 1 by
simply setting its type to 0 and forwarding the route description,
class 1 does not provide any confidentiality of the content of an
option.
3.1.3 Format
The format field specifies the format of the data included after the
option header. Formats:
0 none, no data present.
1 one or more 32-bit signed integers
2 a character string, null terminated
3 one or more real numbers
4 an octet string
5 one real, followed by a character string
Format is also orthogonal to type, but a particular type is usually
only reasonably represented by one format. This allows decoding of
all option values for logging and other troubleshooting, even when
the option type is unknown. (A new unknown format will still present
a problem.)
Format 4, octet string, is to be represented in dotted-decimal byte
form when printed; it is normally an internet address.
Format 5 is intended for dimensioned parameters with the character
string giving the dimension or scale.
3.2 Metrics and Options
As much as possible, metrics are kept in the base units of bytes and
seconds, by analogy to the physics systems of MKS (meter-kilogram-
second) and CGS (centimeter-gram-second) of base units.
Bytes aren't the real primitive, the bit is. We are thus using a
multiple of 8 that isn't part of what one would come to expect from a
decimal metric system that uses the other prefixes. However, since K
(kilo) is often taken to be 1024, and M (mega) to be 1,048,576 (or
even 1,024,000) we allow this liberty.
Distance is measured in units also unique to the field. It is the
integer number of times that a datagram must be forwarded to reach
the destination. (Hop count.)
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RFC 1476 RAP June 19933.2.1 Distance
The Distance metric counts the number of hops on a route; this is
included in the RAP route command header.
The initial distance at insertion into the RAP domain by the origin
of the route MUST be less than or equal to 2z, where z is the number
of zero bits in the route mask.
If the origin derives the route from RIP or OSPF, and the distance
exceeds 2z, the route must not be used.
When a router originates a route designed to permit aggregation, the
distance is usefully set to more than 0; this allows simple subset
aggregation without propagating small distance changes repeatedly as
the internal diameter of the described network changes.
For example, for routers designated to announce a default route for
an AD, with a 24/48 mask, the maximum initial distance is 96.
3.2.2 Delay
The Delay metric (Type = 2) measures the one-way path delay. It is
usually the sum of delays configured for the gateways and interfaces,
but might also include path segments that are actually measured.
Format is real (3), with one value. The units are seconds.
3.2.3 MTU
The MTU metric (Type = 3) measures the minimum value over the route
of the Maximum Transmission Unit, i.e., the largest IP datagram that
can be routed without resulting in fragmentation.
Format is one integer, measuring the MTU in bytes.
3.2.4 Bandwidth
The Bandwidth metric (Type = 4) measures the minimum bandwidth of the
path segments that make up the route.
Format is one real, representing bandwidth in bytes/second.
3.2.5 Origin
The origin attribute (type = 5) identifies the router that originally
inserted the route into the RAP domain. It is one of the IP
addresses of the router, format is 4.
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RFC 1476 RAP June 19933.2.6 Target
The target attribute (type = 6) identifies a host or network toward
which the route should be propagated, regardless of proximity
filtering that would otherwise occur. This aids in the establishment
of tunnels for hosts or subnets "away from home." It can be used to
force the route to propagate all the way to the home network, or to
try to propagate a better route to a host that the origin has
established a connection (e.g., TCP) with. Note that a router can
distinguish these two cases during proximity filtering by comparing
the route described with the host or network identified by the target
option.
Format is 4.
3.2.7 Packet Cost
The packet cost metric (type = 7) measures the actual cost (to
someone) of sending data over the route. It is probably either class
3 or 0. Format is 5.
The real number is the cost in currency units/byte. Tariffs set in
packets or "segments" should be converted using the nominal (or
actual path) size. For example, Sprintnet charges for DAF
connections within its network are US$1.40/Ksegment thus for segments
of 64 bytes, the cost is 0.000021875 USD.
The string is the 3 capital letter ISO code [ISO4217] for the
currency used. Funds codes and codes XAU, XBA, XBB, XBC, XBD, and
XXX are not used.
If a route already has a packet cost in a different currency
associated with it, another instance of this option should be added.
RAP implementations MUST NOT attempt to convert the currency units
except when actually making a route selection decision. That is, the
effects of a currency conversion should never be propagated, except
for the proper effect of such a selection decision.
3.2.8 Time Cost
The time cost metric (type = 8) measures the actual cost of holding
one or more paths in the route open to send data. It is probably
either class 3 or 0. Format is 5.
The real number is the cost in currency units/second. For example,
Sprintnet charges for international connections (to typical
destinations) are US$10/hour so the cost is 0.002777778 USD.
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RFC 1476 RAP June 1993
The other notes re codes used and conversions in the previous section
also apply.
3.2.9 Source Restriction
A source restriction option (type 9, format 4, class 2 or 3)
indicates that a route may only be used by datagrams from a
particular source or set of sources. The data consists of a network
or host number, and a mask to qualify it. If multiple source
restriction options are included, the restriction is the logical
union of the sources specified; i.e., any are permitted.
Source restrictions must be added to routes when the RAP system has
security filters set in the IP forwarding layer. This is necessary
to prevent datagrams from taking "better" routes that end in the
datagram being silently discarded at the filter. Note that this
propagates confidential information about the security configuration,
but only toward the net authorized to use the route if the RAP
implementation is careful about where it is propagated.
3.2.10 Destination Restriction
A destination restriction option (type 10, format 4, class 3) serves
only to provide a non-contiguous mask, the destination already having
been specified in the command header. Data is the destination
network and mask.
3.2.11 Trace
Trace (type 11, format 4, class 0) provides an indication that the
route has propagated through a particular system. This can be used
for loop detection, as well as various methods of troubleshooting.
The data is one internet address, one of the addresses of the system.
If an arriving route already carries a trace identifying this system,
and is not an update, it is discarded. If it is an update, the route
is purged.
Trace SHOULD NOT be simply added to every route traversing a system.
Rather, it should be added (if being used for loop detection) when
there is a suspicion that a loop has formed.
When the distance to a destination has increased twice in a row in a
fairly short period of time, and the number of trace options present
in the route did not increase as a result of the last update, the RAP
process should add a trace option identifying itself to the route.
Effectively, when a loop forms, one router will select itself to be a
tracer, adding itself and breaking the loop after one more turn. If
that fails for some reason, another router will add its trace. Each
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RFC 1476 RAP June 1993
router thus depends in the end only on its own trace and will break
the loop, even if the other routers are using other methods, or
simply counting-out the route.
3.2.12 AUP
The AUP (Acceptable Use Policy) option (type 12, format 2, class
any), tags a route as being useable only according to the policy of a
network. This may be used to avoid traversal of the net by (for
example) commercial traffic, or to prevent un-intentional use of an
organization's internal net. (It does not provide a security barrier
in the sense of forwarding filters, but does provide cooperative
exchange of information on the useability of a net.)
The data is a domain name, probably the name of the network, although
it may be the name of another organization. E.g., the routers that
are subject to the NSF AUP might add NSF.NET as the descriptor of
that policy.
3.2.13 Public
Public (type 13, format 0, class 2 or 3) marks the route as
consisting in part of a public broadcast medium. Examples of a
public medium are direct radio broadcast or a multi-drop cable in
which other receivers, not associated with the destination may read
the traffic. I.e., a TV cable is a public medium, a LAN within an
organization is not, even if it can be easily wiretapped.
This is intended for use by cable TV providers to identify routes
that should not be used for private communications, in spite of the
attractively high bandwidth being offered.
4. Procedure
Routing information arrives in the RAP process from other peers, from
(local) static route and interface configuration, and from other
protocols (e.g., RIP). The RAP process filters out routes that are
of no interest (too detailed or too "far away" in the topology) and
builds an internal database of available routes.
From this database, it selects routes that are to be active and loads
them into the IP forwarding database.
It then advertises those routes to its peers, at a greater distance.
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RFC 1476 RAP June 1993
-------------------------------------------------------------------
[incoming routes]
|
v
[proximity filtering/aggregation] [static routes]
| |
v v
[route database] ---> [selected active routes]
^ |
| v
[RIP, etc. routes] [output filtering]
|
v
[routes advertised]
-------------------------------------------------------------------
4.1 Receiver filtering
The first step is to filter out offered routes that are too "far
away" or too specific. The filter consists of a maximum distance at
which a route is considered usable for each possible (contiguous)
mask.
Routers that need universal connectivity must either pass through the
filter all routes regardless of distance (short of "infinity"), and
use aggregation to reduce them, or have a default route to a router
that does this.
The filter may be adjusted dynamically to fit limited resources, but
if the filter is opened, i.e., made less restrictive, there may be
routes that have already been offered and discarded that will never
become available.
4.2 Update of metrics and options
The process then updates any metrics present on the route to reflect
the path to the RAP peer. MTU and bandwidth are minimized, delay and
cost are added in. Distance is incremented. Any unknown options
cause class-dependent processing: discarding the option (class 2) or
route (3), or marking the route as non-propagatable (1).
Policy options that are known may cause the route to be discarded at
this stage.
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RFC 1476 RAP June 19934.3 Aggregation
The next step is to aggregate routes that are subsetted by other
routes through the same peer. This should not be done automatically
in every possible case. The more information that is propagated, the
more effective the use of forward route identifiers is likely to be,
particularily in the case of aggregating into a default route.
In general, a route can be included in an aggregate, and not
propagated further, if it is through the same peer (next hop) and has
a smaller distance metric than the containing route. (Thus datagrams
will always travel "downhill" as they take more specific routes.)
The usual case of aggregation is that routes derived from interface
configurations on the routers from which they originated are subsumed
into routes offered by routers explicitly configured to route for an
entire network, area, or AD. If the larger area becomes partitioned,
unaggregatable routes will appear (as routes outside the area become
the shortest distance routes) and traffic will flow around the
partition.
Attributes of routes, particularily policy options, may prevent
aggregation and may result in routes simply being discarded.
Some information about aggregation also needs to be represented in
the forwarding database, if the route is made active: the router
will need to make a decision as to which forward route identifier to
use for each datagram arriving on the active route.
4.4 Active route selection
The router selects those routes to be entered into the IP forwarding
database and actively used to forward datagrams from the set of
routes after aggregation, combined with routes derived from other
protocols such as RIP. This selection may be made on any combination
of attributes and options desired by local policy.
4.5 Transmitter filtering
Finally, the RAP process must decide which routes to offer to its
peers. These must be a subset of the active routes, and may in turn
be a selected subset for each peer. Arbitrary local policies may be
used in deciding whether or not to offer any particular route to a
given peer.
However, the transmitter must ensure that any datagram filters are
represented in the offered route, so that the peer (and its peers)
will not route into a black hole.
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RFC 1476 RAP June 19934.6 Last resort loop prevention
RAP is designed to support many different kinds of routing selection
algorithms, and allow them to interact to varying extents. Routes
can be shared among administrations, and between systems managed with
more or less sophistication.
This leaves one absolute requirement: routing loops must be self-
healing, regardless of the algorithm used on each host. There are
two caveats:
1. A loop will not fix itself in the presence of an error that
continually recurs (thus re-generating the loop)
2. The last resort algorithm does not provide rapid breaking of
loops, only eventual breaking of them even in the absence of
any intervention by (human) intelligence.
The algorithm relies on the distance in the RAP route header. This
count must be updated (i.e., incremented by one) at each router
forwarding the route.
Routers must also impose some limit on the number of hops permitted
in incoming routes, discarding any routes that exceed the limit.
This limit is "infinity" in the classic algorithm. In RIP, infinity
is 15, much too low for general inter-domain routing.
In RAP, infinity is defined as 2z + i, where z is the number of zero
bits in the mask (as described previously) and i is a small number
which MUST be configurable.
Note that RAP depends on the last resort algorithm, "counting to
infinity," much less than predecessors such as RIP. Routes in the
RAP domain will usually be purged from the net as the purge route
command is flooded without the delays typical of periodic broadcast
algorithms. Only in some cases will loops form, and they will be
counted out as fast as the routing processes can exchange the
information.
5. Conclusion
Unlike prior routing protocols, RAP is designed to solve the entire
problem, from hands-off autoconfiguration of LAN networks, to
implementing the most complex policies of international carriers. It
provides a scaleable solution to carry the Internet forward to a
future in which essentially all users of data transmission use IP as
the fabric of their networks.
Ullmann [Page 18]